Distance from Epicenter

Calculate your distance from an earthquake epicenter and check if you would have felt the shaking.

Calculation

距震中距离如何影响震感

震中是地球表面正对震源（即断层破裂起始点）上方的点。地震波从震源向各个方向辐射传播，其振幅由于几何扩散和衰减（岩石和土壤对能量的吸收）而随距离减小。这就是为什么远离震中时震感强度会减弱，不过减弱速率取决于当地地质条件、地震深度和波的类型。

有感半径——人能感知地震的最大距离——主要取决于震级和深度。浅层M6.0地震可能在200-400公里外被感知，而深度300公里的深层M6.0地震尽管震级相同，有感半径可能更小。浅层地震将能量集中在地表附近，产生更强但更局部的震动，而深层地震将能量分散到更广泛但强度较低的区域。

关键概念

半正矢公式计算球面上两点之间的大圆距离，这是确定震中距的方法。
地震波类型——P波（纵波，压缩波）最先到达且传播最快；S波（横波，剪切波）其次到达并造成更多破坏；面波（勒夫波和瑞利波）最后到达但携带最大的破坏性能量。
烈度衰减模型（如USGS ShakeMap使用的模型）预测给定震级和深度下修正麦加利烈度如何随距离递减。
场地放大效应可导致软土上的远距离地点比基岩上的近距离地点经历更强烈的震动。

常见用途

快速判断您的位置是否在已报告地震的有感区域内。
为应急规划场景估算特定距离处的震动强度。
了解深度和震级如何相互作用产生不同的震动范围。

How to Use

1
Set the Epicenter Coordinates
Enter the earthquake epicenter latitude and longitude, or search by a recent event name. Epicenter coordinates are published by USGS, EMSC, and national seismological agencies within minutes of a significant event.
2
Enter Your Location
Provide your current location as coordinates, a city name, or an address. The tool calculates the great-circle (surface) distance using the Haversine formula.
3
Interpret Your Shaking Estimate
Review your estimated Modified Mercalli Intensity (MMI) and whether the shaking would likely be felt. The estimate uses USGS ShakeMap attenuation relations and assumes average soil conditions.

About

The relationship between distance and ground shaking follows well-defined attenuation functions central to seismic hazard analysis. As seismic waves travel outward from the hypocenter, their amplitude decreases due to geometric spreading (energy distributed over an ever-larger spherical surface) and anelastic attenuation (energy absorbed as heat by imperfectly elastic rock). These effects are codified in Ground Motion Prediction Equations (GMPEs), empirical models derived from thousands of recorded earthquakes that predict median and standard deviation of ground motion parameters—such as peak ground acceleration (PGA) or spectral acceleration—as functions of magnitude, distance, depth, and site class.

ShakeMap, developed by the USGS and now adopted by agencies worldwide, combines recorded ground motions from seismograph networks with GMPE predictions to produce near-real-time maps of shaking intensity across a region. The maps use the Modified Mercalli Intensity (MMI) scale, where MMI I–II represents not-felt or barely-felt shaking, MMI V causes objects to fall from shelves, MMI VII–VIII damages poorly constructed buildings, and MMI X–XII represents near-total structural destruction. ShakeMaps are generated within minutes of significant earthquakes and are used immediately by emergency managers for resource deployment decisions.

For coastal regions, distance from a submarine epicenter carries additional significance beyond ground shaking: earthquake-generated tsunamis. Tsunamis are most efficiently generated by thrust earthquakes on shallow-dipping (< 30°) submarine faults with vertical displacement components greater than roughly 1 meter. The 2004 Sumatra tsunami was triggered 250 km offshore Aceh; wave heights at the coast reached 30 m. Coastal residents within 100 km of a subduction zone should be familiar with the Drop-Cover-Hold guidance for shaking, followed by immediate vertical or inland evacuation upon feeling prolonged shaking lasting more than 20 seconds.

FAQ

How is distance from an earthquake epicenter measured?

Seismologists distinguish between epicentral distance (the surface distance from a location to the point directly above the focus) and hypocentral distance (the straight-line distance to the actual rupture source, accounting for depth). For shallow earthquakes (< 20 km depth), the two values are nearly identical at distances greater than a few kilometers. Epicentral distance is measured along the Earth's surface as the great-circle arc between two points and is expressed in degrees (1° ≈ 111 km) in seismological contexts, or in kilometers for engineering applications. Hypocentral distance is the appropriate quantity for attenuation relations used to predict ground motion.

How far away can an earthquake be felt?

Felt distance depends primarily on magnitude, depth, and regional geology. As a rough guide: a M4.0 may be felt up to 100 km away; a M6.0 up to 400–500 km; a M8.0 can be felt across continental scales exceeding 1,500 km. Deep earthquakes (> 300 km) can be felt at exceptional distances because their waves travel through the mantle with lower attenuation than crustal paths. The 2013 Okhotsk Sea M8.3 deep earthquake (610 km depth) was felt across Russia and as far as Moscow (6,000+ km). Geological structure also plays a role: the central US has lower attenuation than the western US, meaning eastern earthquakes historically felt over larger areas than western events of similar magnitude.

What is the difference between epicenter and hypocenter?

The hypocenter (also called the focus) is the actual point within the Earth where the rupture initiates—where elastic strain energy is first released along the fault. The epicenter is the geographic point on the Earth's surface directly above the hypocenter, obtained by projecting vertically upward. USGS and other agencies report both: epicenter coordinates (latitude/longitude) and focal depth. Depth matters greatly for hazard: a M6.5 earthquake at 5 km depth can be highly destructive, while the same magnitude at 600 km depth may be barely felt at the surface. The 2001 Bhuj earthquake (India) had a shallow depth of 23 km and caused catastrophic damage; in contrast, deep subduction events in the Tonga trench at 600+ km rarely cause surface damage.

Does earthquake depth affect felt shaking?

Earthquake depth has a major influence on the distribution and character of felt shaking. Shallow earthquakes (< 20 km, called 'crustal' earthquakes) concentrate energy near the surface and produce intense, short-duration shaking in a relatively small area. As depth increases, the energy radiates over a larger spherical surface, reducing peak intensities but spreading felt shaking over broader regions. Intermediate-depth earthquakes (70–300 km) in subducting slabs can shake large areas at lower intensities. Very deep earthquakes (> 300 km, 'deep focus') are felt across enormous distances but rarely cause significant damage. The 1994 Northridge M6.7 at 19 km depth caused 57 deaths and US$20 billion in damage; its shallow depth was a key factor in the severity.

Can I determine my distance from an earthquake using a seismograph?

Yes—this is the classical method of locating earthquakes. Seismographs record P-waves (compressional, faster, ~6 km/s in the crust) and S-waves (shear, slower, ~3.5 km/s). The time interval between the P-wave and S-wave arrivals, called the S-P time or Wadati plot, is directly proportional to the distance: distance ≈ S-P time × Vp × Vs / (Vp − Vs). A simple rule of thumb: for every 8 seconds of S-P interval, the earthquake is approximately 80 km away. Three or more seismograph stations allow triangulation of the epicenter. Modern seismic networks with hundreds of stations achieve epicenter location accuracies of 1–5 km within seconds of a significant event.